It is known in the art to detect the position of an actuation element, for example a finger of a user contactless. For this it is provided for that a high frequency sensor signal is generated by means of a signaling device. This signal is capacitive coupled into a user sitting on a seat of the vehicle. The device of the signal sensor therefore preferably is provided at the seat of the vehicle. In order to detect an approach of a finger of the user towards a surface sensors are arranged at or surrounding the surface. When the finger approaches these sensors, then a capacity changes in the signal circuit comprising the signaling device, the user and the sensors, which in turn are connected and coupled, respectively, to the signaling device via the vehicle. The change in the capacity, which may be considered as a change in a received signal strength of the sensor signal transmitted via the body of the user, is a measure for a distance of the finger of the user to the respective sensor. Other sensor systems may be designed following other principles, however, in each case a signal strength of the HF signal transmitted via the user is evaluated. By means of the known positions of the sensors and the separate sensor signals, which in each case indicate a measure for a distance of the body part to the respective sensor, the position of the body part may be determined by means of a triangulation.
The basic principles of such a HF signal transmission via the body of a user in a motor vehicle for contactless detecting an approach of a body part are described in WO 2004/078536.
Position determination units are often used in motor vehicles in connection with display and control devices which are arranged in a center console between a driver's seat and a co-driver's seat. In particular, the position determination units are used together with such display and control devices which comprise a display surface. The position determination unit is then used in order to determine an approach of a finger of a user relative to the display surface. Because of the device being arranged between the driver's seat and the co-driver's seat an approach of the finger, however, as a general rule does not occur perpendicular with respect to the display surface. Rather, the approach in most cases occurs in an inclined angle. Since the sensors are arranged surrounding the display surface or spaced therefrom not always the finger is the body part which is proximate closest to one of the sensors.
Shown in FIGS. 1a to 1d are four situations in which a user in each case approaches his right index finger 1 towards different contact positions 2 to 5 of a display surface 6 designed as a touch-screen and arranged to the right of the user. Arranged surrounding the display surface 6 are sensors 8 to 11 at respective corner positions 7. During the contact situation of FIG. 1a in which the user touches the contact position 2 located in the left lower corner using the right index finger 1 none of the sensors 8 to 11 is affected interferingly by another body part in a noteworthy manner.
During an approach of the right index finger 1 to the contact position 3 located in the left upper corner of the display surface 6, however, the sensor 8 arranged at the left lower corner is additionally affected by a hand 12 and a forearm 13 of the user.
During the situation of FIG. 1c in which the user approaches the right index finger 1 to the contact position 4 arranged in the right upper corner the sensor 8 arranged in the left lower corner is strongly affected by the hand 12 and the forearm 13.
During the situation depicted in FIG. 1d in which the right index finger 1 of the user is approaching the contact position 5 located in the lower right corner the sensors 8 and 11 respectively arranged at the left lower corner and the right lower corner are also affected by the hand 12 and the forearm 13. Therefore, a triangulation carried out by means of the sensor signals often does not result in an accurate position determination of the right index finger 1 used to make contact.